What does the notation [x] mean in this question?

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Taken from the Rhode Island Mathletes Documentation from 1986 (I encountered this problem while scanning through older documents from a contest I'm currently preparing for)

if $[x]$ is the largest integer not bigger than $x$, what is $[-2.4] + [2.4]$

I'm not sure what this question is asking at all, for a start, how can $[x]$ be less than $x$? And what do the brackets mean at all?

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1 Answer

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This is the floor function, which takes any number and "rounds down" to the closest integer that's smaller than or equal to $x$. The ceiling function is very similar, but "rounds up" to the closest integer greater than or equal to $x$. Note that if $x$ is whole, $\lfloor x \rfloor$ = $x$, not $x-1$.

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